Sorry if this has been asked before, I can't find the answer online. The "user_jvm_args.txt" explains how to do it somewhat, but I don't know where to put that line into. The txt file says this:
 
	# Xmx and Xms set the maximum and minimum RAM usage, respectively. 
	# They can take any number, followed by an M or a G. 
	# M means Megabyte, G means Gigabyte. 
	# For example, to set the maximum to 3GB: -Xmx3G 
	# To set the minimum to 2.5GB: -Xms2500M
 
	# A good default for a modded server is 4GB. 
	# Uncomment the next line to set it. 
	# -Xmx4G
 
	Thanks!
 
	 
 
	Normally, I would just add the xml and xms into a jar file, but later versions of Forge don't have the jars.